The present invention relates to steam turbines and, in particular, to control of steam turbine-generators during generator loading following a cold startup.
When a steam turbine-generator is started from a cold start, it takes a substantial time to generate a full operating steam supply. During early stages of startup, the generator is brought up to synchronous speed and loaded lightly. Synchronous speed is defined as a speed at which the frequency and phase of the electricity produced by the generator matches that of the network or load to which it will be connected. After synchronous speed is attained, the electrical output of the generator is connected to the load and loading toward full output can begin. While ramping up the loading, the steam turbine may require a greater steam flow than the boiler is capable of supplying. When this happens, the steam pressure may decline.
One type of control system programs the loading of the turbine-generator by comparing a commanded megawatt output of the generator with the actual output of the generator to produce an error signal. This error signal is used, either directly or after further processing, to control steam valves feeding steam admitted to the turbine. As noted above, during the startup and loading process, less than the full operational supply of steam is available from the boiler. Accordingly, it is possible that, steam pressure perturbations may occur during loading. If a sharp downward swing occurs in the steam pressure, the control system sharply opens the steam valves to compensate by attempting to increase the supply of steam to the turbine. However, since a limited supply of steam is available, rapidly opening the steam valves, instead of increasing the steam fed to the turbine, further reduces the available steam pressure. The reduced steam pressure causes the error signal to open the steam valves even wider, which, due to the limited steam supply available, further reduces the steam pressure. These effects tend to reinforce each other as positive feedback to increase the instability of the system.